Usain Bolt threw himself onto a couch with a broad smile but that vanished when the Jamaican sprint superstar was quizzed about doping by the international media on Thursday.

"All I hear in the last weeks is doping, doping, doping," he told a packed news conference in a Beijing hotel two days ahead of the start of the world championships in the iconic Bird's Nest stadium.

Bolt, who turns 29 Friday, had earlier said he was back in great shape and ready to further raise his title collection of so far 14 golds at worlds and Olympics since he rose to stardom in the same stadium at the 2008 Games.

But he could simply not run away from the fact that athletics and the ruling body IAAF has been under fire over various doping allegations and that his principle rival Justin Gatlin is a two-time doping offender who returned in 2010 from a four-year ban.

Bolt refused to be drawn into a good v evil debate around the blue riband 100 metres which starts Saturday and has its final Sunday night.

"It is not my call if you can get back into the sport," Bolt said in the direction of those who think Gatlin should not be able to run any more. "I go by the rules but I don't worry. I try to compete and get things done.

"I am running for myself. People say I have to win for the sport. But it is not just me. It is the responsibility of all athletes that the sport can go forward."

Bolt himself says he is ready to go forward and back in top shape after two modest seasons with some injury problems. His Beijing preparations were also hampered by a leg problem and Bolt's season best is 9.87 seconds from last month in London.

Gatlin, 33, by contrast, leads the world list this year with personal bests 9.74 seconds over 100m and 19.57 in the 200m.

But Bolt is not discouraged and believes he will rise to the occasion like in the past although his world records from 2009 of 9.58 and 19.19 will likely not be under threat.

"I am in great form. I am running well, my start is coming together at the right time. I am ready to go," he said.

"I would have liked to have had more races before the championships. But I have had good training. My coach (Glen Mills) is happy and when he is happy I know it is good."

Leading Gatlin 6-1 in their series and returning to the site of his 2008 glory will further boost Bolt after some frustrating weeks.

"I am definitely happy to be back. I think they (the Chinese) still love me," he said.

Bolt is fully focussed, there will be no big birthday party to celebrate his 29th on Friday, and he has also ditched his chicken nuggets obsession from seven years ago for a healthier diet including meat, regular chicken dishes and rice this time around in Beijing.

After all, Bolt says he has matured over the years although the attention he catches as the undisputed face of the sport remains the same.

"I have changed a lot and have learnt a lot over the years. I am a different and much improved man," Bolt said.